This project will explore morphologic and functional events defining sequential steps in differentiation of B cells. We will attempt to determine whether the pre-B cell is the locus for generation of antibody diversity. Pre-B cells have been defined in mouse, man and rabbit as the earliest cell to appear during ontogeny. They are identified by the presence of intracellular IgM and the lack of detectable surface IgM. In adult mammals they reside exclusively in the bone marrow. Other studies concern the regulation of the "switch" from IgM synthesis to production of other classes during B cell differentiation induced by lipopolysaccharide. Having determined that expression of C gamma genes in cells bearing membrane IgM does not require DNA synthesis, while restriction of IgG production is dependent upon several proliferative cycles, we will study the role of inhibitors of transcription in this process. We hope to learn whether synthesis of 2 immunoglobulin classes by single immunocytes requires dual transcription, as opposed to the possibility that persistent IgM synthesis is maintained by a long-lived mRNA.